1. Field of the Invention
This invention relates to programmably configuring an integrated circuit or use with a bus.
2. Description of the Prior Art
Integrated circuit (IC) processors, including for example microprocessors, microcontrollers, and digital signal processors, typically communicate addresses and data to and from external devices over multi-conductor uses. These buses typically include 8, 16, 32 or 64 conductors in the case of data uses, and perhaps 20 to 30 conductors in the case of address buses, depending on memory size. The number of conductors in a bus is referred to as the bus "width". In many cases, the data bus and the address bus are separate. In other cases, the addresses are time-multiplexed with the data on the same bus conductors. It is desirable in some cases for a processor to be able to interface with more than one bus width. For example, a microcontroller may interface with an 8-conductor data bus in a low-cost system, or may alternatively interface with a 16-conductor data bus in a higher-performance system. However, it is usually necessary to instruct the processor prior to system start-up as to what type of bus it must interface with, in order to avoid corrupted data and/or incorrect instructions that could lead to system failure.
Configuring the processor so as to interface with the chosen bus type has been accomplished using integrated circuit terminals that are connected to a high or low voltage source (V.sub.DD or V.sub.SS). However, that technique typically requires extra integrated circuit terminals that serve no other function. It is also known that processors can be configured using external pull-up or pull-down resistors connected to terminals that serve a dual-use function. For example, the terminals may be sampled during an initial configuration period at system power-up, and then used as input or output terminals after the initial configuration period. However, that technique requires space for the configuration resistors on a printed circuit board. Such space is increasingly at a premium as fine pitch packages and various flip-chip technologies lead to finer conductor spacing. In addition, the use of external resistors may complicate in-board circuit testing, as via JTAG (IEEE Standard 1149.1), and results in power dissipation during system activity. It is known also to use an initial configuration period for setting the control signal modes; for example, active logic high or alternatively active logic low.